test/decode/tinyjpeg: make sure the pointer is valid before dereferencing it

Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>
Reviewed-by: Sean V Kelley <seanvk@posteo.de>
(cherry picked from commit 8455834161bab3374fe9756fd4a28d919027daf7)